Automatic fidelity control



atented Mar. 28, 1939 UNITED STATES AUTGMATIC FIDELITY CONTROL Harold A. Wheeler, Great Neck, N. Y., assignor to Hazeltine. Corporation, a corporation of Delaware Application .nine 1s, 1931, sei-iai No. 148,893

12 Claims.

This invention relates to modulated-carrier signal receivers and, more particularly, to signalselecting systems of such receivers and methods of, and means for, controlling the fidelity and selectivity of reproduction thereof to discrim- 1 inate against undesired signals.

It is frequently dimcult in receivers of the type described to reproduce a desired signal without undesired signal interference such as w is caused by signals on carrier frequencies near the desired signal carrier and by static and other so-called background noises which are ordinarily present at the higher modulation frequencies. In order, therefore, to obtain faithful reproduction of the desired signal, substantially free from undesired signal interference, it isk necessary to utilize selective systems which are effective to pass a band of desired signal frequencies sumciently narrow to attenuate greatly the undesired relatively high-frequency signal interference.

Heretofore various selective systems have been devised for this purpose, adaptable for use in either the radio-frequency or audio-frequency 25 channel of the receiver. Certain of such selective systems have included means for automatically adjusting the width of the selected band of frequencies so as to contract the band only in the presence of undesired signals of sufficient amplitude to cause appreciable interference or noise. In the absence of interference, the width of the selected band is expanded sufficiently to admit and pass all of the useful signal frequencies, thus procuring the highest fidelity of reproduction.

While such systems as those heretofore used have proved generally satisfactory, they have, however, been subject to certain disadvantages. For example, the frequency bands which they have been capable of passing have been relatively narrow with respect to their mean frequency and a critical cutoi has been extremely difficult to obtain. Where a selector system is used in an audio-frequency channel, both of these considerations are important since it is necessary `to pass a relatively very wide band of frequencies and it is essential that frequencies higher than those of the desired band be greatly attenuated.

It s an object of the present invention to provide animproved signal-selector system for passing a band of frequencies with substantially uniform gain for all frequencies within the band and with substantial attenuation of frequencies higher than the highest frequency of the band.

55` It is a further object of the invention to provide an improved signal-selector system adjustable to vary the width of the band of frequencies passed thereby without substantially altering the gain of the system.

In vaccordance with a preferred embodiment 5 of the invention, there is provided a signalselector system for passing a band of frequencies comprising aperiodic frequency-discriminating terminal circuits. Separate directive coupling means are provided for individually coupling the circuits in the forward and backward directions. The coupling means are substantially less frequency-discriminative than the terminal circuits and co-operate with these circuits to provide a coupling reaction therebetween which is degenerative over the main portion of the band-. and regenerative at the higher and lower frequencies of the band and which i's also preferably degenerative at frequencies higher than the highest frequency of the band. Means are also provided for simultaneously so adjusting the coupling means relative to each other as to adjust the width of the band of frequencies passed by the system without substantially altering the gain of the system.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing, Fig. 1 is a schematic diagram of a complete modulated-carrier signal receiver embodying one form of the present invention, while Fig. 2 is a group of curves illus- 35 trating certain operating characteristics of the system shown in Fig. 1 to aid in the understanding of the invention.

Referring now more particularly to Fig. 1, there is shown schematically a superheterodyne receiver embodying a radio-frequency amplifier l0 having its input circuit connected with an antenna Il and ground I2. Connected in cascade to the output circuit of the amplifier lll, in the order named are an oscillator-modulator or frequency changer I3, an intermediate-frequency amplifier and band-pass selector system i4, a detector and automatic amplification control or A. V. C. rectifier l5, a'n audio-frequency amplifier and selector system I6, additional stages of audio-frequency amplification ll, and a loudspeaker IS. Suitable connections I9 and I9' are provided for supplying an A. V. C. potential negatively and positively, respectively, to the control grids of one or more of the tubes of the preceding stages of the system. The stages Ill-I5, inclusive, and I1 may be of any suitable conventional construction and are shown in schematic form for the purpose of brevity. The output clrcuit of the detector I5 may include a conventional direct current blocking condenser 20 and voltage divider resistance 2I, by adjustment of which the volume level output of the receiver may be controlled in the usual manner.

Preferably, the amplifier and selector I4 includes an adjustable band-pass selector of a suitable design for automatically controlling the width of the band of frequencies passed thereby in accordance with received signal conditions in order to reduce undesired signal disturbances at relatively high modulation frequencies. This control is preferably effected by the application of a unidirectional control-bias voltage to control electrodes of the tubes of this system. In order to develop a suitable biasing voltage for this purpose. as well as for controlling the audio-frequency amplifier and selector system I6, as will be hereinafter explained, there is connected to the output circuit of the intermediate-frequency amplifier I4 a frequency detector 22 having a rectlfier 23 coupled to its output circuit. Part of the output voltage of rectier 23 is applied, by way of a lead 2l including a filter resistor 25, to a terminal of the amplifier and selector system Il.

The details of the selector system Il, as well as the detector 2l and rectifier 23, constitute no part of the present invention and, therefore, are not described herein. For a detailed description of the construction and operation of suitable apparatus for these stages, reference is had to applicant's copending application, Serial No. 148,894, filed concurrently herewith, for improvements in Automatic selectivity and fidelity control. The system of the present invention, also described and illustrated in'that copending application but not claimed therein, is embodied in the audiofrequency amplifier and selector system I6 and will be hereinafter described in detail.

Neglecting for the moment the novel features of the amplifier and selector system I6, the system as thus far described includes the features of a superheterodyne receiver with automatic selectivity control. Since the operation of such a receiver is well understood in the art, a detailed explanation thereof is considered unnecessary. Briefly, however. signals intercepted by the antenna II are selected and amplified by the radiofrequency amplifier I and converted into intermediate-frequency signals in a conventional man'- ner in the oscillator-modulator I3. The intermediate-frequency signals are thereupon selected and amplied by the amplifier and selector system I4 and translated to the detector and A. V. C. rectifier I5, wherein the audio-frequency signal and unidirectional A. V. C. voltages are derived. The audio-frequency signal is further selected and amplified by the audio-frequency stages I6 and I1 and translated therefrom to the loudspeaker I3 for reproduction. The unidirectional voltages developed by the A. V. C. rectifier are applied negatively and positively to control electrodes of tubes in the preceding stages of the system, thereby to maintain the signal output amplitude of the intermediate-frequency amplifier and selector system Il within a relatively narrow range for a wide range of signal input amplitudes. While the detector I detects only amplitude modulation of the received carrier, the desired signal being represented by such modulation, undesired signal disturbances cause both f amplitude and frequency modulation of the carrier and the detector 22 detects only the modulation of the latter type. This detected frequency modulation is rectified by the rectifier 23 to provide a unidirectional control-bias voltage which varies in accordance with the undesired signal disturbances. The amplifier and selector system I4 is controlled by the unidirectional bias voltage developed by the rectifier 23 so as to decrease the width of the band of frequencies passed by the system in response to received undesired signal disturbances, thereby to reduce the effects of such disturbances.

Referring now more particularly to the parts of the receiver embodying the present invention, the audio-frequency amplifier and selector system I 6 comprises frequency-discriminating input and output terminal circuits which are substantially aperiodic within the pass band of the selector. The aperiodic input circuit is connected across the output section of voltage divider 2I and includes, in series, a resistor 28, a condenser 30, a resistor 3|, and an audio-frequency by-pass condenser M, and a condenser 29 is connected in parallel with the elements 30, 3|, and M. The output circuit includes a primary winding 32 of an audio-frequency transformer 33, the secondary winding 34 of which is coupled to the additional audio-frequency amplifier stages I1. A trap circuit comprising an inductor 35 and a condenser 36, in series, is connected across the winding 32. The trap circuit 35, 36 is tuned to l0 ,kilocycles, the usual frequency difference between carriers o n adjacent channels, so that it leaves the output circuit also aperiodic at the lower frequencies, within the band passed by the system.

Coupling the terminal circuits, just described, in the forward direction is a vacuum tube having its input circuit coupled across resistor 3l and its output circuit coupled across the winding 32. Coupling the terminal circuits in the backward direction is a vacuum tube 21 having its input circuit coupled across a portion of winding 32 through the grid condenser 31 and its output circuit coupled to the input circuit of the system by way of a resistor 38 in its cathode lead, the audio-frequency voltage across which is coupled to the input circuit of the tube 26 through a high resistor 39, across condenser 29 and in series with condenser 30. The directive coupling means comprising the tubes 26 and 21 and their couplings to the terminal circuits are substantially nondiscriminative and, therefore, are less frequency-discriminative than the terminal circuits. Operating potentials are supplied to the electrodes of the tubes 26 and 21 from suitable sources, as indicated by the batteries 40.

For the purpose of controlling the selector and amplifier system I6 in accordance with the presence of undesired signal interference, unidirectional voltage developed by the rectifier 23 is applied negatively to the control grids of the tubes 26 and 21 by way of connections 4! and 42,v

respectively, the connection 4I including a suitable lter comprising a series resistor 43 and a shunt condenser 44 and the connection 42 including a filter comprising a series resistor 45 and a shunt condenser 46, connected to grid leak 41.

Referring now to the operation of the audiofrequency amplifier and selector I6, over the major part ofthe audio-frequency range a degenerative feedback is provided. This relationship is obtained by means of the phase reversal of the signal which is effected by the forward the backward coupling tube 21. is coupledto the input circuit, no phase reversal of the feed-back voltages is effected thereby. The impedance elements in the input and output circuits are so proportioned and related that they produce no substantial resultant phase shift ofthesignal voltages of frequencies within the major middle portion of the frequency range which the system is designedv to pass. Therefore, the resultant feed-back voltage is reversed in phase with respect to the input Voltage at such frequencies.

At the lower frequencies of the audio-frequency range a regenerative feedback is effected. This is due primarily to the transformer primary winding 32, the voltage across which leads the anode current of tube 26 by approximately 90 degrees, and to the series condenserY 30 which at the lower frequencies has an impedance comparable with that of resistors 3l and 39 combined and draws a leading current which develops across resistor 3l a feed-back voltage having a leading phase angle approaching 90 degrees with respect to the feed-back voltage across resistor 38. Thus, these two 90 degree leading phase displacements, together with the 180 de-` gree phase shift effected by the forward coupling tube, results in a feed-back voltage tending to be in phase with the input voltage applied to the grid of tube 26 so that the system is regenerative at the lower frequencies.

Also, at the higher frequencies of the audio band and below the resonant frequency of the trap circuit, a regenerative feedback is effected. This is due primarily to the trap circuit 35, 36

' and the parallel condenser 29. At these higher frequencies below the trap frequency, the load impedance of the tube 26 is determined primarily by the trap circuit 35, 36 which is capacitive at frequencies lower than the resonant frequency of the trap, so that the voltage thereacross lags the anode current of tube 26 by a phase angle approaching degrees. A portion of this voltage, derived from the winding 32, is applied to the grid of the backward coupling tube 21. Also, at these higher frequencies, the reactance of condenser 29 is comparable with that of resistors 28 and 2|, in series, or that of condenser 30 and resistor 3l, in series, so that a substantial part of the current` due to the voltage derived from resistor 38 flows through condenser 29. However, resistor 39 maintains this current nearly in phase with the voltage across resistor 38, so that the feed-back voltage across condenser 29 lags that across resistor 38 by an angle approaching 90 degrees. Thus, the resultant feed-back signal undergoes a phase reversal in tube 26 and two additional shifts approaching 90 degrees, lagging so that it tends to be in phase with the input voltage, andthe system is regenerative.

f Frequencies higher than the trap frequency are, for practical purposes, outside of the useful range of audio frequencies and it is, therefore, desirable that voltages at these frequencies be attenuated. The trap circuit "is inductively reactive at' these frequencies and, therefore, shifts the feed-back voltage by an angle approaching 90 degrees leading, rather than lagging coupling means are substantially less discrimiand co-operate with these terminal circuits to provide a coupling action therebetween which is degenerative over the major portion of the band passed by the system and at frequencies higher than the highest frequency of this band, and is regenerative at the higher and lower frequencies of the band. The system, therefore, when properly adjusted, passes the frequencies of the desired band with substantially uniform gain while greatly attenuating undesired frequencies higher than the-highest frequency of the band.

'Ihe adjustment of the selectivity of the amplifier and selector system I6 may best be explained with reference to the curves shown in Fig. 2 which illustrate the operating characteristics of this system and in which the abscissae represent frequencies in cycles, and the ordinates y indicate relative gain of the system in Voltage ratio. Curve A represents the normal condition of operation, or maximum fidelity of reproduction, which is obtained with maximum feedback, while curve B represents the condirelatively narrow and includes 'a sharp dip to t zero at 10 kilocycles, representing infinite attenuation at the resonant frequency of the trap circuit. Curve B indicates that the system not only is relatively sharply selective, so as to reduce the signal disturbances at the higher frequencies below the resonant trap frequency, but also attenuates the frequencies higher than the highest frequency of the band. v

Ihe transition from the condition represented by curve B to that represented by curve A results partly from the adjustment of the feed-back effect and partly from adjustment of the relative .transconductances of the forward and backward coupling tubes 26 and 21; that is, while the feedback obtained is dependent only. on the product of the forward and backward transconductances, the gain of the system is dependent not only on this product, but also on the ratio of these two transconductances. If then, under the conditions assumed above, feedback were effected merely by increasing the transconductance of the backward coupling tube 21, the effect would be to lower the frequency-gain characteristic in the middle of the range,`and also above l0 kilocycles, and to. raise the curve slightly at the lower frequencies and the higher frequencies below 10 kilocycles. Curve C represents the condition which would then be obtained. y w,

In accordance with the present invention, however, the width 'of the band is adjusted by decreasing `:the negative bias on both the forward and backward tubes simultaneously and in such proportionrelative to each other that the resultant increase in forward transconductance holds the gain unifo'rm in the middle of the range in spite of the degenerative feedback caused by increasing the backward and incidentally the forward transconductances. Increasing both the forward and backward transconductances at the same rate would result in a slight increase in gain.

`native 'than the terminal circuits ofthe system- Hence, the forward transconductance is preferably increased at a slightly lower rate. While there may be exceptions to thisrule, when a wide range of feed-back effect is required, no dimculty is likely to be experienced in determining, for any case, what relative variations of the negative-bias voltages are required for the two tubes. It is to be noted that the manner in which the feedback is made degenerative, not only in the middle of the band but also above 10 kilocycles, is relatively simple; that is, the use of the trap circuit in the output circuit of the system provides the'desired effect. Condenser 29 and trap circuit 35, 38 may be interchanged if desired. If trap circuits, however, were connected both in the input and in the output circuits, the advantage would be lost since the feedback would then be regenerative at frequencies above, as well as just below, the trap frequency. i

While there has been described what is at present considered the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from this invention, and, therefore, it is aimed in the appended claims to cover all such changes and modications as fall within the true spirit and scope of this invention.

What is claimed is:

l. A selector system for passing a band of frequencies comprising frequency-discriminating terminal circuits substantially aperiodic within the pass band of the selector, directive coupling means coupling said circuits in one direction, separate directive coupling means coupling said circuits in the other direction, said coupling means being substantially less frequency-discriminative than said terminal circuits and cooperating with said terminal circuits to provide a coupling reaction therebetween which is degenerative over the major portion of said band and is regenerative at the higher and lower frequencies of said band, and means for simultaneously so adjusting said coupling means relative to each other as to adjust the width of the band of frequencies passed by said system without substantially altering the gain of the system.

y 2. A selector system for passing a band of frequencies comprising frequency-discriminating terminal circuits .substantially aperiodic within the-pass band of the selector, directive coupling means coupling said circuits in one direction and separate directive coupling means coupling said circuits in the other direction, said coupling means being substantially less frequency-discriminatlve than said terminal circuits and cooperating with saidterminal circuits to provide a coupling reaction therebetween which is degenerative over the major portion of said band. is regenerative at the higher and lower` frequencies of said band, and is degenerative at frequencies higher than the highest frequency of said band.

3. A selector system for passing a band of frequencies comprising frequency-discriminating terminal circuits substantially aperiodic within the pas band oi' the selector, directive coupling means coupling said circuits in bne direction and separate directive coupling means coupling said circuits in the other direction, .said coupling means being substantially less frequency-discriminative than said terminal circuits and cci-operating with said terminal circuits to provide a coupling reaction therebetween which is degenerative over the major portion of said band, is regenerative at the higher and lower frequencies of said band, and is degenerative at frequencies beyond one of the limiting frequencies of said band.

4. A selector system for passing a band of frequencies comprising frequency-discriminating terminal circuits substantially aperiodic within the pass band of the selector, directive coupling means coupling said circuits in one direction and separate directive coupling means coupling said circuits in the other direction, said coupling means being substantially less frequency-discriminative than said terminal circuits and co-operating with said terminal circuits to provide a coupling reaction therebetween which is degenerative over the major portion of said band, is regenerative at the higher and lower frequencies of said band, and is degenerative at frequencies higher than the highest frequency of said band, and means for simultaneously so adjusting said coupling means relative to each other as to adjust the width of the band of frequencies passed by said system without substantially altering the gain of the system.

5. A selector system for passing a band of frequencies comprising frequency-discriminating terminal circuits substantially aperiodic within the pass band of the selector, one of said circuits comprising resistance and reactance elements, directive coupling means for coupling said terminal circuits in one direction and separate directive coupling means coupling said terminal circuits in the other direction, said elements of said terminal circuits being of such types and values that they co-operate with said coupling means to provide a coupling between said terminal circuits which is degenerative over the major portion of said band and at higher frequencies than the highest frequency of said band and is regenerative at the lower and higher frequencies of said band.

6. A selector system for passing a band of frequencies comprising frequency-discriminating terminal circuits substantially aperiodic within the pass band of the selector including resistance and reactance elements, coupling means for coupling said terminal circuits in one direction and separate coupling means coupling said terminal circuits in the other direction, said elements of said terminal circuits being of such types and values that they cooperate with said coupling means to provide a coupling between said terminal circuits which is degenerative over the major portion of said band and at higher frequencies than the highest frequency of said band and is regenerative at the lower and higher frequencies of said band, and means for adjusting said coupling means to adjust the band-pass characteristic of said system.

'1. A selector system for passing a band of frequencies comprising an aperiodic input circuit including a resistance-reactance network, an aperiodic output circuit including an impedance network effective to develop voltages displaced in phase with respect to the output current at higher and lower frequencies of said band, forward coupling means coupling said circuits and effective to reverse the phase of the output current with respect to the input voltage at frequencies passed thereby, separate backward coupling means coupling said output circuit to said resistance-reactance network, said resistance-reactance network being effective to develop therein a voltage displaced in phase with respect to the voltage derived from said output circuit at said higher and lower frequencies and said coupling means being thereby without substantially altering the gain of the system.

8. A selector system for passing a band oi frequencies comprising frequency-discriminating terminal circuits substantially aperiodic within the pass band of the selector, directive coupling means coupling said circuits in one direction and effective to cause a phase reversal of the signal passed by said system and separate directive coupling means coupling said circuits in the other direction, said coupling means being substantially less frequency-discriminative than said terminal circuits, a pair of phase-shifting means each effectiveto displace the phase of the signal by an angle approaching 90 degrees leading at the lower frequencies of said band, and a second pair of phase-shifting means each effective to displace the phase of the signal by an angle approaching 90 degrees lagging at the higher frequencies of said band, whereby the coupling between said terminal circuits is degenerative over-the major portion of said band and is regenerative at the higher and lower frequencies of said band.

9. A selector system for passing a band of frequencies comprising frequency-discriminating terminal circuits substantially aperiodic within the pass band of the selector, directive coupling means coupling said circuits in one direction and effective to cause a phase reversal of the signal passed by said system and separate directive coupling means coupling said circuits in the other direction, said coupling means being substantially less frequency-discriminative than said terminal circuits, a pair of phase-shifting means each effective to displace the phase of the signal by an angle approaching 90 degrees leading at the lower frequencies of said band, and a pair of phase-shifting means each effective to displace the phase of the signal by an angle approaching 4 90 degrees lagging at the higher frequencies of said band, one of said last-mentioned means being a trap circuit also effective to displace the phase of the signal by an angle approaching 90 degrees at frequencies beyond one of the limiting frequencies of said band and in a direction opposite to that of the other of said last-mentioned means, whereby the coupling between said terminal circuits is degenerative over the major portion of said band and at said frequencies beyond 5 one of the limiting frequencies of said band and is regenerative at the higher and lower frequencies of said band.

10. A selector system i'or passing a band of frequencies comprising an input circuit including a resistance-reactance network, an output circuit comprising an impedance network including a resonant trap circuit and en'ective to develop voltages displaced in phase with respect to the output current, in one sense at lower frequencies of said band and at frequencies higher than the highest frequency of said band. and in an opposite sense at higher frequencies of said band, forward coupling means coupling said circuits and effective to reverse the phase of the output cura rent with respect to the input voltage at frequencies passed thereby, and separate backward coupling means coupling said circuits. said resistance-reactance network being effective to develop therein voltages displaced in phase with respect to the voltage derived from said output circuit in one sense atsaid lower frequencies of said band and in an opposite sense at said higher frequencies of said band and at frequencies higher than the highest frequency of said band, said two coupling means being substantially nonfrequen'cydiscriminative.

ll. A selector system for passing a band of frequencies comprising an input circuit including a resistance-reactance network, an output circuit comprising an impedance network including a resonant trap circuit and eifective to develop voltages displaced in phase with respect to the output current, in one sense at lower frequencies of said band and at frequencies higher than the highest frequency of said band, and in an opposite sense at higher frequencies of said band, forward coupling means coupling said circuits and effective to reverse the phase of the output current with respect to the input voltage at frequencies passed thereby and separate backward coupling means coupling said circuits, said resistance-reactance network being eifective to develop therein voltages displaced in phase with respect to the voltage derived from said output circuit in one sense at said lower frequencies of said band and in an opposite sense at said higher frequencies 'of said band and at frequencies higher than the highest frequency of said band, said two coupling' means being substantially nonfrequencydiscriminative. and means for adjusting said coupling means toadiust the width of the band of frequencies passed thereby .without substantially altering the gain of thesystem;

l2. A selector systemfor passing a band o'f fre- A quencies comprising an'. input circuit including a resistance-reactance network, an output circuit comprising an impedance network including 'a trap circuit sharply responsive at the highest frequency of said band and being effective to develop voltages displaced in phase with respect to the output current in a leading direction at lower frequencies of said band and at frequencies higher than the highest frequency of said band, and in a lagging direction at higher frequencies of said band, forward coupling means coupling said circuits and eective to reverse the phase of the 'output current with respect to the input voltages at frequencies passed thereby, and separate backward coupling means coupling said circuits, said resistance-reactance network being effective to develop therein voltages displaced in phase. with respect to the voltages developed by said output circuit, in the same sense as the phase displacements effected by said outputcircuit at said higher and lower frequencies of said band and in an opposite sense to the phase displacement effected by' said output circuit at said frequencies higher than the highest frequency of said band, said coupling means being substantially non-frequencydiscriminative.

HAROLD A. 

